Chip card comprising an antenna

ABSTRACT

The invention concerns an RF-type contactless chip card, comprising a sensor coil ( 1 ), an electronic circuit ( 2 ) connected to the coil and an antenna ( 3 ) made form a material with high magnetic permeability, including a first part located substantially in the plane of the coil upper surface (A), a second part located substantially in the plane of the coil lower surface (B) and a connecting part (C) located in the center of the coil. The latter part is dimensioned relative to the antenna ( 3 ) such that the voltages induced at its terminals are of the same order of magnitude, whether the antenna is parallel or perpendicular to the lines of flux generated by a reader.

FIELD OF INVENTION

The present invention concerns a contactless smart card, of the RF(radio-frequency) type.

BACKGROUND OF INVENTION

Numerous embodiments of this type of card are available on the market.They generally include an electronic module including a sensor coilassociated with an integrated circuit, this coil allowing, on the onehand, RF signals to be transmitted, and on the other hand, the powernecessary for the electronic module to operate to be supplied. U.S. Pat.No. 4,115,091 and European Patent No. 762 535 disclose examples of suchembodiments. The simplest cards comprise only a code which can be readremotely, but more and more read/write smart cards can also be foundincluding an EEPROM type memory capable of saving its data even in theabsence of a power source. Writing in such memories requires arelatively large amount of power which has to be taken across theterminals of the sensor coil. This may pose problems, particularly in <<hands free >> access systems which constitute one of the majorapplications of such cards.

One solution for obtaining the power necessary for the card to operateat the greatest possible distance is to use a coil with a larger surfacearea allowing a maximum of flux to be picked up. Thus, round coils oftenhave a diameter close to the width of the card and, in certain cases,rectangular coils are even used which cover practically the entiresurface of the card. Another possibility consists in integratingantennas made of materials with high permeability in the card in orderto concentrate the flux passing in proximity to the card at the centreof the sensor coil. The document WO. 98/52141 discloses such a systemwhich allows coils of small diameter to achieve performances equivalentto those obtained with cards of large diameter.

However, a major problem remains. Indeed, in both the aforementionedcases, the maximum flux in the coil is only obtained when the card has awell defined orientation relative to the lines of flux generated by thereader. When the position of the card is changed, the flux quicklydecreases and there are even numerous neutral positions where the fluxin the coil is quite simply zero or, at least, close to zero. Thisobviously poses serious problems, particularly within the scope of <<hands free > applications since the position of the card relative to thelines of flux generated by the reader may be very variable dependingupon the person carrying it and the place where he puts his card.

BRIEF SUMMARY OF INVENTION

The object of the present invention is precisely to provide a sensorcoil/antenna combination allowing the aforementioned problem to belargely resolved by making the quantity of flux picked up by the coilmuch more independent of the relative position between the card and thelines of flux generated by the reader.

More precisely, the invention concerns a contactless RF-type smart card,including a sensor coil, an electronic circuit connected to the coil andelements made of material with high magnetic permeability acting asantenna, including a first part located substantially in the plane ofthe upper coil surface and a second part located substantially in theplane of the lower coil surface. It is characterised in that the antennaincludes a connecting part between the first and second part, positionedat the centre of the coil, the latter being dimensioned relative to theantenna such that the voltages induced across its terminals are of thesame order of magnitude, whether the antenna is parallel orperpendicular to the lines of magnetic flux generated by a readerdevice.

BRIEF DESCRIPTION OF SEVERAL VIEW OF THE DRAWINGS

The invention will be better understood upon reading the followingdescription, given by way of explanation and with reference to theannexed drawings, in which:

FIGS. 1a, b and c show several embodiments of a smart card according tothe invention with its antenna and its sensor coil;

FIGS. 2a, b and c show the voltage across the terminals of the sensorcoil as a function of the position of the latter in the flux for a firstantenna shape; and

FIGS. 3a, b and c show the voltage across the terminals of the sensorcoil as a function of the position of the latter in the flux for asecond antenna shape.

DETAILED DESCRIPTION OF THE INVENTION

Reference will be made first of all to FIG. 1 which shows schematicallya smart card according to the invention, in a credit card format. Thisis the most common format, but the invention is, of course, applicableto smart cards and other electronic tags of very varied dimensions.

The smart card shown includes a sensor coil 1 connected to an electroniccircuit 2 including a memory and RF transmission means at 125 kHz, afrequency which is used very generally in this type of application. Thistype of circuit, well known to those skilled in the art, may be, forexample, one of the circuits marketed by the EM Marin company(Switzerland) under the references H 4000, 4001 to 4005 and 4050.

An antenna 3 made of material with high magnetic permeability passesthrough coil 1. This antenna has to be very thin and have a particularshape. Thus, it may be made in a strip or a sheet of metal with a highnickel content such as Mumétall® or Permalloy® which are reputed to havevery high magnetic permeability. Sheets which are 100 microns, or even50 microns thick can be found on the market. This small thickness isnecessary, on the one hand, in order not to significantly increase thethickness of the card and, on the other hand, to reduce eddy currentlosses in the antenna. Another possibility for further decreasing suchlosses consists in making the antenna in a stack of even thinner sheets,for example 10 or 20 microns, electrically insulated from each otherlike transformer plates.

The coil 1, circuit 2 and antenna 3 assembly is incorporated into thethickness of the body of card 4, for example by overmoulding with aplastic material.

Antenna 3 includes 3 zones, zone A which is in the plane of the uppercoil surface, zone B which is in the plane of its lower surface and zoneC which is a connecting zone between zones A and B. The over-thicknessdue to the antenna is limited to the thickness of zones A and B whichare superposed on each of the coil surfaces. For an antenna thickness of50 microns, there is thus an over-thickness of 100 microns. One mayeasily adjust the thickness of the coil to compensate for thisover-thickness and keep the total thickness of the card within standardvalues.

In the example of FIG. 1a, antenna 3 is placed diagonally. It isstraight and, since its width is markedly less than the inner diameterof coil 1, it may easily, be slid inside. It is thus made in a singlepiece and zone C is a simple fold which allows the difference in heightbetween parts A and B to be compensated for.

In the absence of an antenna, the magnetic flux parallel to the surfaceof the smart card do not pass through coil 1 and thus do not generateany voltage across its terminals. As described in the document WO98/52141, the fact of incorporating antenna 3 in the card enables thesemagnetic flux parallel to the surface of the card to be picked up, moreparticularly those which are along the antenna axis. These flux willpass from zone a, i.e. from the upper surface of coil 1, to zone B, i.e.towards the lower surface thereof, or vice versa, passing through thecentre of coil 1 in zone C. This allows the voltage necessary forelectronic circuit 2 to operate properly to be generated across theterminals of coil 1.

In the aforementioned Swiss Patent, the antenna picks up practicallynone of the flux perpendicular to the surface of the smart card, and itsuse is clearly associated with readers which generate lines of fluxparallel to said surface. With such a structure, the antenna essentiallyallows the flux to be concentrated at the centre of the card, in a coilof small diameter, but in no way resolves the problem of neutralpositions.

The present invention thus constitutes a significant improvement, mainlyfor << hands free >> applications insofar as it largely resolves thisproblem of neutral positions. By associating a conventional coil 1 oflarge diameter with antenna 3, as shown in FIG. 1, the fluxperpendicular to the surface of the smart card pass directly through thecentre of coil 1 without passing through antenna 3. With an equivalentdiameter, equivalent features to those of a conventional smart cardwithout an antenna are obtained. Thus, by combining these two systems,an antenna plus a coil of large diameter, one can make pass through thecoil both the flux parallel to the surface of the card, via antenna 3,and the flux perpendicular to said surface, directly through largediameter coil 1, which is entirely consistent with the desired object ofeliminating neutral positions, as will appear more clearly hereinafter.

FIG. 1b shows a card wherein antenna 3 is no longer placed diagonally,but is aligned along the longitudinal axis of the smart card. Indeed,the response of the card in different positions can be modified bychanging the antenna configuration, which enables the operation of thecard to be optimised as a function of its application. In the case ofFIG. 1b, the antenna is again made in a single piece. The features ofthis card are given in FIG. 2.

In FIG. 1c however, antenna 3 is a more complex Z-shape, allowing thefeatures to be modified, as shown in FIG. 3. It may be easier to make itin two parts. The upper part A and the lower part B overlap in zone Cand are bent so as to superpose each other. They may also be assembledby riveting, bonding, or any other method. One may also, in certaincases, have an antenna in two parts and leave a gap between them,without this being detrimental to the proper working of the system.

The voltage induced at 125 kHz across the terminals of sensor coil 1with the antenna configuration of FIG. 1b is shown in FIG. 2. Axis X isin the direction of the length of the card, axis Y is in the directionof the width and axis Z is perpendicular to the first two, i.e.perpendicular to the surface of the card. The voltage is representedvectorially, as a function of the angle of rotation of the antennaassuming that it rotates about an axis perpendicular to the plane of thedrawing. The fine line represents the voltage obtained without anantenna, and the thicker line the voltage obtained with the antenna inplace. The starting position, corresponding to the angle of rotation 0,is shown to the left. Measurements are taken inside a Helmoltz coilpowered by a constant current so as to have a uniform flux. Thedirection of this flux is horizontal, as indicated by the arrow.

In FIG. 2a, axes X and Y are in the plane of the drawing and the cardrotates about axis Z. The coil is parallel to the lines of flux over theentire 360 degrees of rotation. These lines of flux do not thereforepass through the coil so that, without an antenna (dotted curve), theinduced voltage is practically zero whatever the angle.

With the antenna, the flux passing through the antenna inside the coilis maximum at 0 and 180 degrees when the antenna is parallel to thelines of flux. It is zero at 90 and 270 degrees when the antenna isperpendicular to the lines of flux. There is thus a sinusoidaldistribution of the induced voltage as a function of angle. Distributionwith the antenna, even if it has two zero crossovers, is in any eventmuch more advantageous than distribution without an antenna.

In FIG. 2b, axes Z and X are in the plane of the drawing and the cardrotates about axis Y. The coil is parallel to the lines of flux at 0 and180 degrees. It is perpendicular to the lines of flux at 90 and 270degrees. Without an antenna (dotted curve), the flux is zero in thefirst case and maximum in the second. There is thus sinusoidaldistribution of the induced voltage as a function of angle, with zerocrossovers at 0 and 180 degrees, and maximum at 90 and 270 degrees.

With the antenna, the flux passing through the antenna inside the coilis maximum at 0 and 180 degrees when the antenna is parallel to thelines of flux. It is zero at 90 and 270 degrees when the antenna isperpendicular to the lines of flux. A very interesting phenomenon maythen be observed. At 0 and 180 degrees, all of the flux passing throughthe coil is brought by the antenna and there is a corresponding inducedvoltage. At 90 and 270 degrees, the antenna is inoperative, but there isthe voltage due to the direct passage of the flux through the coil,corresponding to the maximum voltage obtained when there is no antenna.In the intermediate positions, for example at 60 and 240 degrees, theflux passing through the coil directly, on the one hand, and through theantenna, on the other hand oppose each other, so that there is a certainreduction in the induced voltage, without however, there being a zerocrossover. Conversely, at 140 and 330 degrees, these flux are addedtogether, so that this is where the maximum induced voltage is found.

In FIG. 2c, axes Z and Y are in the plane of the drawing and the cardrotates about axis X. The coil is parallel to the lines of flux at 0 and180 degrees. It is perpendicular to the lines of flux at 90 and 270degrees. Without the antenna (dotted curve), the flux is zero in thefirst case and maximum in the second. There is thus sinusoidaldistribution of the induced voltage as a function of angle, with zerocrossovers at 0 and 180 degrees, and maximums at 90 and 270 degrees.

In that case, the antenna is practically inoperative at all angles, sothat practically the same induced voltage distribution is found aswithout an antenna. In such case, it is thus the use of a relativelylarge coil which is advantageous, whereas the antenna adds nothing. Itwill be noted that, without an antenna, the voltage across the terminalsof the coil is proportional to the square of its diameter. With a smallcoil, the voltage would be negligible whatever the angle. In order tohave similar performances to the smart card in the main directions, itis thus necessary to dimension the sensor coil with respect to theantenna such that the induced voltage across its terminals is of thesame order of magnitude, whether the antenna is perpendicular orparallel to the lines of flux.

It can thus be seen clearly that by combining, according to the presentinvention, a relatively large coil which picks up the lines of fluxperpendicular to the smart card which are not picked up by the antenna,whereas the latter picks up the lines of flux parallel to the smart cardwhich are not picked up by the coil, this condition is achieved andneutral positions are considerably reduced, which cannot be achievedeither with a coil alone, even a coil of large diameter, or with anantenna associated with a small coil.

In FIG. 3, the same curves are shown, but with a Z-shaped antenna. InFIG. 3a, axes X and Y are in the plane of the drawing and the cardrotates about axis Z. In FIG. 3b, axes Z and X are in the plane of thedrawing and the card rotates about axis Y. Finally, in FIG. 3c, axes Zand Y are in the plane of the drawing and the card rotates about axis X.This Z-configuration of the antenna is particularly advantageous insofaras it has practically no neutral position when one rotates about axis Z(FIG. 3a). When one rotates about axes Y or X (FIGS. 3b and 3 c), theflux in the antenna is zero in certain positions, but the flux whichpasses directly through the coil without passing through the antennacompensates for this. It can be seen that there is thus very homogenousdistribution of the induced voltage in the three cases, with practicallyno zero crossover.

It is thus possible to adapt the features of the smart card, and moreparticularly its behaviour in the different positions which it mighthave with respect to the lines of flux generated by the reader system byadapting the shape of the antenna working in association with a sensorcoil of large diameter.

Of course, other possible configurations of the smart card according tothe present invention exist, but the description thereof would not addany new elements to the description of the invention.

What is claimed is:
 1. A contactless radio frequency smart card,including a sensor coil (1) having at least two coil terminals, anelectronic circuit (2) connected to said coil (1) and an antenna (3)made of a material with high magnetic permeability, the antenna (3)including a first part (A) located substantially in the plane of theupper coil surface and a second part (B) located substantially in theplane of the lower coil surface, characterised in that said antenna (3)includes a connecting part (C) between the first part (A) and the secondpart (B), the connecting part (C) passing substantially through thecentre of the coil (1), wherein said coil (1) is dimensioned relative tothe antenna (3) such that the voltages induced across said coilterminals are of the same order of magnitude, whether the antenna isparallel or perpendicular to the lines of flux generated by a reader. 2.A smart card according to claim 1, characterised in that the antenna (3)is made in a thin sheet or strip of metal with high magneticpermeability.
 3. A smart card according to claim 1, characterised inthat the antenna (3) is made of a metal with a high nickel content.
 4. Asmart card according to claim 1, characterised in that the antenna (3)is made in a single piece, the connecting part (C) consisting of a foldpassing through the centre of the coil (1).
 5. A smart card according toclaim 1, characterised in that the antenna (3) is made in two parts, thefirst (A) and the second (B) parts overlapping each other at the centreof the coil (1) in the connecting part (C).
 6. A smart card according toclaim 1, characterised in that the antenna (3) is straight.
 7. A smartcard according to claim 1, characterised in that the antenna (3) isplaced diagonally.
 8. A smart card according to claim 1, characterisedin that the antenna (3) is Z-shaped.